Abstract

The apparent discrepancy in the Hubble constant between local measurements and the prediction of the LCDM model fit to early universe data is one of the most interesting cosmological tensions, perhaps indicating some aspect of our understanding of the universe is missing an important ingredient. I'll give a pedagogical/historical introduction to the development of this tension, and discuss the generic arguments we gave in "The Hubble hunter's guide" for why potential cosmological solutions must involve at least modifications to LCDM just before CMB recombination. I will also discuss challenges to this idea and some of the latest updates on the measurement front. Finally, I'll show some preliminary results from the South Pole Telescope which preview the type of future CMB lensing analysis that can shed further light on this discrepancy.